Koolen didn't like to paint from a photo. Sometimes he used photographs only for clothing if the sitter had less time for the sessions.
Even children he painted preferably live however sometimes it required a lot of patience.
Child with her favorite doll (Maddy Jongen), oil on canvas, 1943
Lucas with ball, oil on canvas, 1953
Child in white dress (Mabel Reesink), Oil on canvas, 1949 (present existence unknown)
Lucas with toy blocks and fruit, oil on canvas, 1952